docs: add Agent Control auth reference

[abhinav-galileo]

Describe your changes

Adds the Agent Control authentication reference page for pluggable management auth, HTTP upstream authorization, namespace scoping, and runtime JWT claims. Also links the existing configuration and authentication how-to docs to the new reference page.

This pairs with agentcontrol/agent-control#214, where the repo README now points to the docs-site page instead of keeping the full auth contract in-repo.

Shortcut ticket

N/A

Checklist before requesting a review

• - Is this ready for review? If not, raise as a draft PR
• - This deployed to a staging environment correctly
• - I have reviewed my changes
• - I have reviewed the deployed version of my changes
• - I have tested any code that is added or updated
• - I have verified all images and videos are clear, with appropriate zoom
• - I have verified all images and videos match production (or dev for unreleased features)
• - I have tested that the content matches the functionality in production (or dev for unreleased features)
• - All checks have passed
• - This references a feature that is public. If not, add a note and we can schedule the merge for after the feature release

Checks run:

• npm run build
• npx --yes markdownlint-cli2 core/authentication.mdx core/configuration.mdx how-to/enable-authentication.mdx

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Project	Status	Preview	Updated (UTC)
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